An optical encoder essentially consists of a code disk that includes a number of uniquely encoded tracks. An example of a conventional code disk is shown in FIG. 1. Code disk 10 includes eight uniquely encoded tracks (rings). Each track consists of various combinations of opaque and transparent locations, arranged so that there is a unique combination of opaque and transparent locations along a radius at each possible angular position of code disk 10.
Code disk 10 rotates in an assembly that also includes a light source and a number of photo-detectors. There is one photo-detector per track, and so in the example of FIG. 1 there would be eight photo-detectors. Light from the light source is directed onto code disk 10. A transparent region on code disk 10 allows light to reach a respective photo-detector while an opaque region prevents light from doing so. The photo-detectors will receive a unique pattern of light depending on the angular position of the code disk 10. Thus, the angular position of code disk 10 can be determined according to which pattern of light is received at the photo-detectors.
Code disk 10 can be used in a position indicating encoder or position feedback encoder to provide closed-loop feedback to a motor control system. For example, an optical encoder incorporating code disk 10 can be used with a robot arm to determine the angular position of the robot arm by reading the pattern of light transmitted by code disk 10. Importantly, this can be accomplished without the need to “home” the robot arm when it is first powered on. In other words, it is not necessary to have the robot arm move to a predefined starting position that provides a reference point for measuring subsequent movements of the arm. Instead, the unique light pattern produced at each angular position can be used to determine the position of the robot arm at any time without the need of a reference point.
The output of the photo-detectors can be used to produce a digital signal. The optical encoder of FIG. 1, with eight tracks on code disk 10 and eight photo-detectors, has a resolution of eight bits and can produce 256 different light or bit patterns per revolution.
One problem with conventional optical encoders is that they are fixed to a limited resolution. To increase resolution, the number of tracks needs to be increased, increasing the size of the code disk. Also, with more tracks, the number of corresponding photo-detectors needs to be increased. Increasing the size of the code disk and the number of photo-detectors will increase the cost of the optical encoder.
Therefore, an optical encoder that can achieve higher resolutions without necessarily increasing the size of the code disk or the number of photo-detectors would be advantageous.